So, in 40K, does it appear that conciousness may partially remain within (powered) cybernetics after the host has otherwise died off?
Short answer, yes.
Long answer:
The universal laws (the cult mechanicum's ten commandments, except there are sixteen) insist that "The Soulless sentience is the enemy of all life", which is the prohibition against A.I. This is why the Kaban project - essentially a fully sentient, completely artificial war machine - was so utterly heretical.
However, this is a prohibition against completely-artificially-initiated sentience. By the time they reach the rank of archmagos, for example, ninety-odd-percent plus of a tech-priest's 'thinking' is happening in electronic cogitators. He is more machine than man but isn't 'artificial' as he's reached that state by a process of steady improvement. He's not soulless because he has been bestowed with a soul at birth by the omnissiah.
The counterpart, a complex machine spirit (say a land raider's weapons cogitator) is soulless in that it's not got a human mind at it's core but nor is it sentient. This is why you don't get star trek-style conversations with the ship's computer; it may be intelligent but it's 'loyal attack dog' intelligent, not sentient.
Where you need more than that, you usually find a real, organic mind - or at least the functional bits of one - wired in. Servitors, cybernetica and thallax all have some portion of a human brain in the decision-making circuit. Titans and warships usually have multiple servitor brains in the system.
With regards to a person surviving within attached augmetics, A
sanctuary of worms isn't the first example. Princeps Macabee - predecessor to Princeps Hekate of
Imperius Dictatio (in the Graphic novel
Titan and mentioned in
Titanicus) died whilst 'plugged in' - and his conciousness survived inside the titan's machine spirit, ultimately saving the engine.